Your queries are natural.
We can not quantify subtle things of life nor we can apply 2 + 2 = 4 formula to them.
We can not get answer if we ask home much love is required to love anyone? People don't see success in yoga, especially in meditation just because of this expectation.
To raise prana or to store it, there is no fixed formula nor you can set any timeline. There is no instrument that can tell you home much you are near to your experience of transformation.
Yes inversion postures do help but it is not end. You have to focus on 3 things
1) Yoga practice
3) Lifestyle changes
But our daily activities in mainstream life are going to inevitably lead to loss of energy in some form
Nature does not care about mainstream life. What you need is detachment from everything so that you can see inward.
Should a practising yogi eat less or eat more?
Moderation is key. More or less shows mental craving to expect result.
Are there specific steps one can take to conserve prana and/or keep on increasing it at least until middle-age?
3 things mentioned above with total detachment and no expectation of result at all. You need to just keep moving as if you will never see result but still you are doing because you love doing it.
energy activation or enlightenment needs good storage of prana". Why? How much?
Concentrated energy is more powerful than scattered. That is why Laser beam is more powerful than other light sources. Raising prana need escape velocity in terms of concentrated prana.
Again how much is absurd question because you can not measure or know how much more is required. You just keep on practicing as if you are storing and when right time comes, you know automatically.
How long does it take to get there?
As I said whenever question of how much and when comes to your mind just remember my example of love ("how much love is required to love anyone or when I will be ready to love anyone")
When you are ready, you know automatically. Your job is to practice and not to set expectations and timelines. Nature has not given those rights to anyone.
What specific asanas/pranayama should one do for this?
Introversion postures are useful but practice others too. Pranayam works wonder in cleansing of energy channels (nadis) so pranayamas are must.